Gold-plating etching process for 5g communication high-frequency signal boards

ABSTRACT

A gold-plating etching process for 5G high-frequency signal boards is carried out according to the following steps: the outer dry film, the plug gold-plating, the film removing, and the alkaline etching The alkaline etching solution comprises 100 to 150 g/L. of cupric chloride, 90 to 120 g/L of ammonium chloride, and ammonia. The pH value is 9.6 to 9.8. The ratio of the ammonia and the alkaline etching solution is (550-800):1000. The present invention provides a gold-plating etching process of 5G high-frequency signal boards. The alkaline etching procedure is performed right after gold-plating, eliminating the outer etching process after gold-plating. Costs of the outer film pressing, the exposure, and the development can be saved. The flow rate is improved. Requirements of 5G communication circuit boards are satisfied. That is, the transmission speed of 5G communication high-frequency signal boards of the present invention is fast.

CROSS REFERENCE TO RELATED APPLICATIONS

This application based upon and claims priority to Chinese PatentApplication No. 101810014782.2, filed on Jan 8, 2018, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a fabrication process forhigh-frequency signal board, particularly to a fabrication process for5G high-frequency communication signal boards.

BACKGROUND

The fifth generation mobile communication standard of mobile phone, alsoknown as the fifth generation mobile communication technology,abbreviated to 5G. It is also an extension of 4G under study. Thenetwork speed thereof is up to 5M/S-6M/S. 5G mainly covers mobilecommunications, Wi-Fi, high-speed wireless data. transmission and otherfields. In terms of user experiences, 5G has a higher rate and a widerbandwidth. The network speed of 5G is expected to be improved to about10 times as that of 4G. A high-definition movie can be downloaded inonly a few seconds. Consumer demands of virtual reality, ultra HD videoand other higher requirements of network experience can be satisfied. Interms of industrial applications, 5G has higher reliability and lowertime delay and can meet the specific needs of industrial applicationssuch as intelligent manufacturing and automatic driving, broadening thespace for the development of integrated industries, and supporting theinnovation and development of the economic society. Therefore, comparedto 4G, 5G requires a faster transmission rate, a lower transmissiontime-delay, and a higher reliability. Thus, the requirements of the 5Gcommunication circuit board are higher transmission speed, lowertransmission time delay, and better reliability.

The signal transmission speed relates to the materials. The scientificpractice has proved that among the commonly used, conductor materialsfor circuit boards, the materials with the best current transmissionperformance are sequentially listed as gold, aluminum, and copper.Therefore, in order to satisfy the transmission requirements, thegold-plating etching process for the entire circuit board is thepreferred requirement to meet the 5G technology.

In the requirements of the traditional gold-plating process, thegold-plating process generally uses the wire to plate. Terminalcustomers do not allow the use of gold-plated wire in the production ofcircuit boards during the gold-plated process, thus, causing manytechnical problems in the production.

In the traditional gold-plating etching process, the gold-plated area ishigher than the non-gold-plated area, meaning that they are not in thesame plane. While the traditional outer dry film etching process isused, the dry film cannot completely fill the connection between thegold-plated area and the non-gold-plated area due to the altitudedifference. The etching solution infiltrates into the junction of goldand copper along the gap between gold and copper during the etching,which results in the etching gap and open circuit. Thus, it is atechnical problem of gold-plating technology. In addition, in thetraditional process, the etching of circuit, after gold-plating, isperformed with acidic etching. The costs of outer film pressing,exposure, development, etching should also be taken into consideration,affecting the transmission rate and reliability of 5G signal board.

SUMMARY OF THE INVENTION

The invention provides a gold-plating etching process for 5Ghigh-frequency signal boards. An alkaline etching process is performedafter the gold-plating, eliminating the outer etching process aftergold-plating. Costs of the outer film pressing, the exposure, and thedevelopment can be saved. The flow speed is improved. The requirementsof 5G communication circuit boards can be satisfied. That is, thetransmission speed of 5G communication high-frequency signal boards ofthe present invention is fast. The transmission delay is low and thereliability is good.

In order to solve the above technical problems, a technical solutionadopted by the present invention is as follows. A gold-plating etchingprocess of 5G communication high-frequency signal boards is provided,wherein the process is performed according to the following steps:

-   -   outer dry film: performing the opening window treatment to the        required circuit and gold-plated areas for gold-plating; making        an entire opening window on the outer layer of the gold-plated        areas;    -   plug gold-plating: before the plug gold-plating, cleaning,        drying, and acid-pickling the copper surface to ensure that the        copper surface does not get oxidized; during the gold-plating        process, performing the nickel-plating at first, and then        performing the activation treatment, and then performing        gold-plating; wherein the gold surface required to be lustrous        and not rough after the gold-plating;    -   film removing: using the sodium hydroxide to remove the film;    -   alkaline etching: using alkaline etching solution as etchant in        alkaline etching; wherein during alkaline etching process, the        circuit and the gold-plating areas are under the protection of        gold after gold-plating; the dry films on the non-gold-plated        areas are stripped off by the film removing process before the        alkaline etching; and the copper surface is exposed to be etched        by the alkaline etching solution;    -   the alkaline etching solution comprises 50-250 g/L of cupric        chloride, 70-150 g/L of ammonium chloride, and ammonia; the pH        value is 9-10; and the temperature of the etching is 50-60° C.;    -   the ratio of the ammonia to the alkaline etching solution is        (550-800):1000.

Further, the fabrication process for the high-frequency signal boardsincludes the following steps: cutting, board baking, inner circuit,electroplating, outer dry film, plug gold-plating, film removing,alkaline etching, optical inspection, solder mask, texting, paneling,electrical testing and visual inspection.

Further, in the plug gold-plating step, the nickel-plating solution usedin the nickel-plating process includes 250-300 g/L of nickel sulfatehexahydrate 35-45 g/L of nickel chloride hexahydrate, 35-45 g/L of boricacid, and brightener; the ratio of the brightener to the nickel-platingsolution is (30-35):1000.

Further, an optical testing machine is used to scan the surface of theboard to detect circuit gaps and pinholes after the step of outer dryfilm and before the step of plug gold-plating.

Further, during the activation treatment, the temperature of theactivation solution is 50-55° C., and the soaking time is 0.5-2 min.

Further, cleaning and drying are performed directly after performing thealkaline etching.

Further, the gold-plating solution used in the plug gold-plating processincludes 2-2.5 g/L of potassium aurocyanide and 0.6-0.8 g/L of platingbath solution. The gold-plating temperature is 50-55° C. The pH value is3.2-3.5.

Further, the concentration of the sodium hydroxide solution in the filmremoving step is 1.0%, and the temperature of the film removing is 38 to42° C.

Further, the alkaline etching solution comprises 100-150 g/L of cupricchloride, 90-120 g/L, of ammonium chloride, and ammonia. The pH value is9.6-9.8.

Further, the ratio of the ammonia to the alkaline etching solution is(670-700):1000.

Further, the temperature of etching is 52-58° C.

The advantages of the present invention are as below.

In the fabrication process of the 5G high-frequency signal boards, thealkaline etching is used instead of the traditional outer etching. Thegold-plating is directly followed by the alkaline etching process. Theability of etching of the alkaline etchant is weak, so that the goldsurface can protect the circuit, eliminating the outer etching processafter gold-plating. Costs of outer layer film pressing, exposure, anddevelopment can be saved. The flow speed is improved. In the presentinvention, the etching gaps and open circuit caused by the infiltrationof the acidic etchant into the junction of the gold and copper on the 5Gcommunication high-frequency signal board can be addressed. Therequirements of the circuit board for 5G communication are satisfied.That is, the 5G communication high-frequency signal board of the presentinvention has the advantages of faster transmission rate, lowertransmission time-delay and, better reliability.

The above description of the present invention is merely an overview ofthe technical solutions of the present invention. In order to clearlyunderstand the technical solutions of the present invention andimplement according to the contents of the specification, the preferredembodiments of the present invention are described in detail below.

DETAILED DESCRIPTION OF THE INVENTION

Specific embodiments of the present invention will be described below byspecific embodiments. Those skilled in the art can easily understand theadvantages and efficacy of the present invention from the contentsdisclosed in this specification. The present invention may be embodiedin other various forms, that is, various modifications and changes maybe made without departing from the scope of the present invention.

Embodiment: a gold-plating etching process for 5G communicationhigh-frequency signal boards, which is carried out according to thefollowing steps:

Outer dry film: the opening window treatment is performed on the desiredcircuit and gold-plated areas for gold-plating. This process isequivalent to the outer positive etching process, making an entireopening window on the outer layer of the gold-plated areas. After outerdry film, an optical inspection machine is used to scan the surface ofboard to detect circuit gaps and pinholes;

Plug gold-plating: before plug gold-plating, the copper surface shouldbe cleaned, dried and, acid-pickling to ensure that the copper surfacedoes not get oxidized. During gold-plating process, the nickel-platingis performed at first. In order to ensure the good adhesive strengthbetween the nickel layer and the copper layer of the high-frequencysignal board, a fresh solution is prepared with pure chemical reagents.In practical production, 10% sulfuric acid or 10% sodium hydroxide isused to adjust the pH value. A small current of 1.0 A is used forelectrolysis. The formula of the nickel-plating solution includes250-300 g/L of nickel sulfate hexahydrate 35-45 g/L of nickel chloridehexahydrate, 35-45g/L of boric acid, and brightener. The ratio of thebrightener to the nickel-plating solution is (30-35):1000. Since nickelis prone to passivation in the air, an activation treatment is performedwithin 1-3 min after nickel-plating. The activation solution is CP40,120 g/L. The solution temperature is controlled at 50-55° C. The soakingtime is about 0.5-2 min. The gold-plating solution includes 2-2.5 g/L ofpotassium aurocyanide and 0.6-0.8 g/L, of plating bath solution. Thetemperature thereof is 50-55° C. The pH value is 3.2-3.5. A smallcurrent of 1.0 A/m² is selected. The gold surface is required to belustrous and not rough after gold-plating.

Film removing: sodium hydroxide is used to remove the film. Theconcentration of sodium hydroxide solution is 1.0%. The film removing isperformed at the temperature of 38-42° C. The edge of the circuit ischecked to make sure whether the film is removed completely after thefilm removing, so as to avoid residual copper on the circuit side orburrs during the alkaline etching process.

Alkaline etching: the etchant used in the alkaline etching process isthe alkaline etching solution. In the alkaline etching process, thecircuit and the gold-plated areas are protected by the gold after thegold-plated areas are gold-plated. The dry films in the non-gold-platedareas are stripped off by the film removing process before the alkalineetching, such that the copper surface is exposed to be etched by thealkaline etching solution.

The alkaline etching solution comprises 100-150 g/L of CuCl₂ (cupricchloride), 90-120 g/L of NH₄Cl (ammonium chloride), and NH₃⋅H₂O(ammonia), wherein the ratio of the ammonia to the alkaline etchingsolution is (670-700):1000, and the pH value of the alkaline etchingsolution is 9.6-9.8. The main chemical reaction equation of alkalineetching is CuCl₂+NH₃→Cu((NH₃))₄Cl₂. The copper on the surface of thehigh-frequency signal board is oxidized by complex ion {Cu(NH₃)₄}²⁺. Theetching reaction equation is Cu+Cu(NH₃)₄Cl₂>2 Cu(NH₃)₂Cl. The produced{Cu(NH₃)₂}¹⁺ does not have the etching ability, but can be rapidlyoxidized by the oxygen in the air in the presence of excess ammonia andchloride ions, so as to produce complex ions {Cu(NH₃)₄}²⁺ having theetching ability. The regeneration reaction equation is2Cu(NH₃)₂Cl+2NH₄Cl+2NH₃+½O₂→2Cu(NH₃)₄Cl+H₂O. CuCl₂, NH₄Cl and NH₃⋅H₂Oare added according to the proportion of etching during the reaction.The etching is performed at the temperature of 52-58° C. The etchingtime and line speed are set according to the thickness of the copper.Generally, the tin stripping section may be performed after etching. Themain ingredient of tin stripping solution is nitric acid, which isstrong acid. Although the gold surface has anti-corrosion property, thegold will be thinned during the reaction. In addition, the appearance ofthe gold will become rough, affecting the gold thickness and appearanceof the finished product. Thus, the tin stripping will not be performedafter alkaline etching. Rather, cleaning and drying processes areperformed to clean the surface of the high-frequency signal board.

The fabrication process of the high-frequency signal board sequentiallyincludes the following steps: cutting, board baking, inner circuit,electroplating, outer dry film, plug gold-plating, film removing,alkaline etching, optical inspection, solder mask, texting, paneling,electric testing and visual inspection.

In the process of cutting, the substrate after being cutting is baked at200° C. for 3 hours, the material of the substrate is chosen FR-4, andthe thickness of the substrate is 0.6 mm.

In the process of board baking, the temperature of the board baking isTg+5° C.-Tg+15° C., the baking time is 1.5 hours to 3 hours. Tg is thecritical temperature at which the raw material melts from a solid stateto liquid in rubbery state. Strict control of the board bakingtemperature and time can effectively prevent the separation of thecoating and the substrate, to further solidify the substrate resin andenhance the binding force between the coating and the substrate.

In the process of the inner circuit, electroless copper plating isconducted on the substrate at first, and then, a layer of wet tiler ispressed on the substrate conducted electroless copper plating. Anegative film is covered on the wet film, then conducting exposure anddevelopment, so that the copper layer at the position of the non-circuitpattern is exposed and etched. After that, the wet film is removed toform the inner circuit. The minimum line width of the inner circuit is0.15 mm and the minimum line-space is 0.13 mm.

In the process of the inner circuit, the light level of exposure islevel 10 to level 11, the speed of the development is 4.5-5.1 m/min, andthe time of the development is 60 s.

In the optical inspection, the gold surface is brighter than the coppersurface since the reflectivity of the gold surface is different fromthat of the copper surface. The grayscale of the optical inspectionmachine needs to be adjusted. The grayscale range of the general copperpanel is 130-170. in order to scan the entire gold-plated board, thegrayscale should be adjusted to 70-120.

In the process of solder mask, a solder mask is brushed on the signalboard, next, baking the signal board brushed with older mask at 70°C.-90° C. for 80 min-120 min at first, and then, baking at 80° C.-90°C., for 45 min-70 min, and filially, baking at 100° C.-150° C., for 45min-70 min. Compared with the prior art, the older mask in the presentinvention is baked and cured in three different temperature stages,including a low temperature stage of 70° C. to 90° C., a mediumtemperature stage of 80° C. to 120° C. and a high temperature stage of100° C. to 150° C., and extending the baking time of low temperaturestage, to improve the solder mask effect and promote product quality.

In the process of solder mask, since the products of customers arehigh-frequency signals, the requirement for signal transmission is high.Noise and crosstalk. cannot be generated in transmission and receptionof signals. Thus, the surface of the circuit cannot be brushed andpolished. Sand blasting and acid pickling processes are used aspre-treatment of the outer layer to clean the board surface.

In the process of electric testing and visual inspection, the test isperformed in accordance with the IPC three-level standard.

The foregoing descriptions are merely embodiments of the presentinvention and are not intended to limit the scope of the presentinvention. All equivalent constructions made by using the contents ofthe present invention, or directly of indirectly applied to otherrelated technical fields are also included in the protection scope ofthe present invention.

1. A gold-plating etching process for 5G communication high-frequencysignal boards, wherein the process comprises the following steps: outerdry film: performing an opening, window treatment on a circuit andgold-plated areas for a gold-plating; making an entire opening window onan outer layer of the gold-plated areas; plug gold-plating: before theplug, gold-plating, cleaning, drying, and acid-pickling a copper surfaceto ensure that the copper surface will not be oxidized; during thegold-plating process, performing a nickel-plating at first, and thenperforming an activation treatment, and then performing a gold-plating;wherein a gold surface is required to be lustrous and not rough afterthe gold-plating; film removing: removing a film with sodium hydroxide;alkaline etching: using an alkaline etching solution as an etchantduring the alkaline etching; wherein during an alkaline etching process,the circuit and the gold-plating areas are under the protection of goldafter gold-plating; stripping off dry films in an non-gold-plated areasby the film removing process before the alkaline etching, such that thecopper surface is exposed to be etched by the alkaline etching solution;wherein the alkaline etching solution comprises 50-250 g/L of cupricchloride, 70-150 g/L of ammonium chloride, and ammonia; a pH value ofalkaline etching solution is 9-10; a temperature of the etching is50-60° C.; a ratio of the ammonia to the alkaline etching solution is(550-800):1000.
 2. The gold-plating etching process for 5G communicationhigh-frequency signal boards according to claim 1, wherein a fabricationprocess for the high-frequency signal boards sequentially comprisescutting, board baking, inner circuit, electroplating, outer dry film,plug gold-plating, film removing, alkaline etching, optical inspection,solder mask, testing; paneling, electric testing and visual inspection.3. The gold-plating etching process for 5G communication high-frequencysignal boards according to claim 1, wherein in the step of pluggold-plating, the nickel-plating solution used in the process ofnickel-plating comprises 250 to 300 g/L of nickel sulfate hexahydrate,35 to 45 g/L of nickel chloride hexahydrate, 35 to 45 g/L of boric acid,and brightener; and a ratio of the brightener to the nickel-platingsolution is (30-35):1000.
 4. The gold-plating etching process for 5Gcommunication high-frequency signal boards according to claim 1, furthercomprising scanning a surface of the 5G communication high-frequencysignal board with an optical testing machine to detect circuit gaps andpinholes after the step of outer dry film and before the step of pluggold-plating.
 5. The gold-plating etching process for 5G communicationhigh-frequency signal boards according to claim 1, further comprisingperforming, cleaning and drying directly after the alkaline etching. 6.The gold-plating etching process for 5G communication high-frequencysignal boards according to claim 1, wherein in the step of pluggold-plating, the gold-plating solution comprises 2 to 2.5 g/L ofpotassium aurocyanide and 0.6 to 0.8 g/L of plating bath solution: atemperature of the gold-plating is 50 to 55 ° C.; and a pH value is 3.2to 3.5.
 7. The gold-plating etching process for 5G communicationhigh-frequency signal boards according to claim 1, wherein in the stepof the film removing a concentration of the sodium hydrate used is 1.0%;and a temperature of the film removing is 38 to 42° C.
 8. Thegold-plating etching process for 5G communication high-frequency signalboards according to claim 1, wherein the alkaline etching solutioncomprises 100 to 150 g/L of cupric chloride, 90 to 120 g/L of ammoniumchloride, and ammonia; and a pH value of alkaline etching solution is9.6 to 9.8.
 9. The gold-plating etching process for 5G communicationhigh-frequency signal boards according to claim 1, wherein a ratio ofthe ammonia to the alkaline etching solution is (670 to 700):1000. 10.The gold-plating etching process for 5G communication high-frequencysignal boards according to claim 1, wherein a temperature of the etchingis 52-58° C.